/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 * <p>
 * http://www.apache.org/licenses/LICENSE-2.0
 * <p>
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.zookeeper.server.quorum;

import org.apache.jute.Record;
import org.apache.zookeeper.server.util.SerializeUtils;
import org.apache.zookeeper.server.util.ZxidUtils;
import org.apache.zookeeper.txn.TxnHeader;

import java.io.IOException;
import java.net.InetSocketAddress;

/**
 * This class has the control logic for the Follower.
 */
public class Follower extends Learner {

    private long lastQueued;
    // This is the same object as this.zk, but we cache the downcast op
    final FollowerZooKeeperServer fzk;

    Follower(QuorumPeer self, FollowerZooKeeperServer zk) {
        this.self = self;
        this.zk = zk;
        this.fzk = zk;
    }

    @Override
    public String toString() {
        StringBuilder sb = new StringBuilder();
        sb.append("Follower ").append(sock);
        sb.append(" lastQueuedZxid:").append(lastQueued);
        sb.append(" pendingRevalidationCount:")
                .append(pendingRevalidations.size());
        return sb.toString();
    }

    /**
     * the main method called by the follower to follow the leader
     *
     * @throws InterruptedException
     */
    /**
     * 和Leader进行网络连接
     */
    void followLeader() throws InterruptedException {
        self.end_fle = System.currentTimeMillis();
        LOG.info("FOLLOWING - LEADER ELECTION TOOK - " +
                (self.end_fle - self.start_fle));
        self.start_fle = 0;
        self.end_fle = 0;
        fzk.registerJMX(new FollowerBean(this, zk), self.jmxLocalPeerBean);
        try {
            //寻找Leader地址
            InetSocketAddress addr = findLeader();
            try {
                //和Leader建立Socket连接
                connectToLeader(addr);
                //向Leader进行注册
                long newEpochZxid = registerWithLeader(Leader.FOLLOWERINFO);

                //check to see if the leader zxid is lower than ours
                //this should never happen but is just a safety check
                long newEpoch = ZxidUtils.getEpochFromZxid(newEpochZxid);
                if (newEpoch < self.getAcceptedEpoch()) {
                    LOG.error("Proposed leader epoch " + ZxidUtils.zxidToString(newEpochZxid)
                            + " is less than our accepted epoch " + ZxidUtils.zxidToString(self.getAcceptedEpoch()));
                    throw new IOException("Error: Epoch of leader is lower");
                }
                //直接和Leader进行数据同步
                syncWithLeader(newEpochZxid);
                //集群启动的磁盘数据恢复，Leader-Follower的数据同步，Leader重新选举之后的数据同步
                QuorumPacket qp = new QuorumPacket();
                //走一个while的死循环
                while (self.isRunning()) {
                    //不停的从Leader那儿去接受同步过来的数据，对数据进行处理
                    readPacket(qp);
                    processPacket(qp);
                }
            } catch (IOException e) {
                LOG.warn("Exception when following the leader", e);
                try {
                    sock.close();
                } catch (IOException e1) {
                    e1.printStackTrace();
                }

                // clear pending revalidations
                pendingRevalidations.clear();
            }
        } finally {
            zk.unregisterJMX((Learner) this);
        }
    }

    /**
     * Examine the packet received in qp and dispatch based on its contents.
     *
     * @param qp
     * @throws IOException
     */
    protected void processPacket(QuorumPacket qp) throws IOException {
        switch (qp.getType()) {
            case Leader.PING:
                ping(qp);
                break;
            case Leader.PROPOSAL:
                TxnHeader hdr = new TxnHeader();
                Record txn = SerializeUtils.deserializeTxn(qp.getData(), hdr);
                if (hdr.getZxid() != lastQueued + 1) {
                    LOG.warn("Got zxid 0x"
                            + Long.toHexString(hdr.getZxid())
                            + " expected 0x"
                            + Long.toHexString(lastQueued + 1));
                }
                lastQueued = hdr.getZxid();
                fzk.logRequest(hdr, txn);
                break;
            case Leader.COMMIT:
                fzk.commit(qp.getZxid());
                break;
            case Leader.UPTODATE:
                LOG.error("Received an UPTODATE message after Follower started");
                break;
            case Leader.REVALIDATE:
                revalidate(qp);
                break;
            case Leader.SYNC:
                fzk.sync();
                break;
        }
    }

    /**
     * The zxid of the last operation seen
     *
     * @return zxid
     */
    public long getZxid() {
        try {
            synchronized (fzk) {
                return fzk.getZxid();
            }
        } catch (NullPointerException e) {
            LOG.warn("error getting zxid", e);
        }
        return -1;
    }

    /**
     * The zxid of the last operation queued
     *
     * @return zxid
     */
    protected long getLastQueued() {
        return lastQueued;
    }

    @Override
    public void shutdown() {
        LOG.info("shutdown called", new Exception("shutdown Follower"));
        super.shutdown();
    }
}
